Thermal ink jet print devices such as a print cartridge, for example, are used to print text and images on a media such as paper. Such devices include thermal ink jet printheads, which comprise nozzle or orifice plates mounted on substrates secured to the body of the print device in communication with a supply of ink in an ink chamber or bladder within the body. Small electric heaters, each in the form of a small resistor in the ink passage at each nozzle, when electrically pulsed, heat the ink which is then expelled as a droplet from the nozzle thereat.
Typical nozzle plate structures are described in U.S. Pat. No. 4,694,308, to C. S. Chan et al, filed Nov. 22, 1985, entitled "Barrier Layer and Orifice Plate for Thermal Ink Jet Printhead Assembly", and U.S. Pat. No. 4,812,859, to C. S. Chan et al, filed Mar. 14, 1989, entitled "Multi-Chamber Ink Jet Recording Head for Color Use", particularly FIG. 6. Both patents are assigned to the assignee of this invention and their teachings are incorporated herein by reference.
A typical thermal ink jet print device comprises a printhead having a silicon substrate structure of glass or monocrystalline silicon on which a silicon dioxide barrier layer is deposited. The individual heater resistors are each deposited on the silicon barrier in an ink passage or priming cavity at each nozzle, individual circuit traces for each resistor provide communication with discrete supplies of electrical energy, for firing the resistors in varying sequences which are orchestrated to print selected characters and images, as is well known. Transfer of resistor heat to the ink boils the ink. The expanding bubble ejects an ink droplet from the nozzle thereat. Resistor heat also heats the silicon substrate structure. During high density printing, such as increasing the number of nozzles being fired and/or resolution, say going from 300 dots per inch to 600 dots per inch, or increasing the firing frequency, the printhead tends to get too hot. Thermal ink jet printhead performance is degraded when the printhead temperature is too high. Temperatures at which print quality degrades vary widely, depending upon the ink jet printhead design.
Thermal ink jet print devices frequently employ a plastic body on which the printhead is mounted. Without the provision of a heat sink, to avoid print quality degradation, a print rate limit has to be determined and not exceeded. Other attempts to solve this overheating problem have included an all metal print device body to conduct the heat away, or a metal fin coupled with air convection cooling. The metal acted like a capacitor or bucket, and once the metal had heated sufficiently, print quality degraded. Convection cooling helped to dissipate the heat, but was expensive and required air velocities that adversely affected ink droplet trajectories which degraded print quality. Reducing the drop ejection frequency lowers the heat flux. This keeps the head cooler. It is also possible to employ various print modes in which the pen scans multiple times over a line to create the desired output. For example, if every other nozzle fired, it would take 2 passes to complete a line, etc. This reduces hard copy throughout.